December 2015 - ghc-tickets

[GHC] #9105: Profiling binary consumes CPU even when idle on Linux.
by GHC 27 Jan '16

27 Jan '16

#9105: Profiling binary consumes CPU even when idle on Linux. ----------------------------+--------------------------------- Reporter: robinp | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Linux Architecture: x86 | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ----------------------------+--------------------------------- The program is main = forever $ threadDelay 1000000 >> return () . Compiled with 32bit GHC 7.6.3 or 7.8.2 on Debian (inside a VM), GHC 7.4.1 on Ubuntu (not VM). The non-profiling binary doesn't consume CPU, the profiling does ~10% (of a 2Ghz machine). Running with +RTS -I0, so this is not the idle gc. When strace-ing, the profiling one seems to receive a constant flow of SIGVTALRM, while the normal receives one burst each second. I see I can switch off "master tick interval" with -V0, and then CPU is not used, but the consequences of this are not very well documented (apart from context switching becoming deterministic). Interestingly, if I compile using profiling on Windows (latest haskell- platform, 64bit), it doesn't use more CPU than the non-profiling. So, the question is, why does this happen on Linux, and if it can be avoided somehow. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9105> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10324: our rts/ghc-prim/base shared library tricks don't work on Android
by GHC 26 Jan '16

26 Jan '16

#10324: our rts/ghc-prim/base shared library tricks don't work on Android -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: Type: feature | Status: new request | Milestone: Priority: low | Version: 7.10.1 Component: Compiler | Operating System: Other Keywords: | Type of failure: None/Unknown Architecture: | Blocked By: Unknown/Multiple | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- I experimented with building dynamically linked executables on Android, and figured I should record my findings somewhere. (I'm interested in dynamic Haskell libraries because I'd like to eventually get ghci on Android, though obviously that would also require a bunch of packaging work!) Building a GHC with dynamic library support was no major problem, and I could build a dynamically linked "Hello, world" executable too. The problem arose when trying to actually run the executable: I got a message to the effect that there were unresolved symbols (that live in libHSrts) when loading libHSbase. I tried relinking the executable to put libHSrts before libHSbase in the list of NEEDED shared libraries, and now I instead got an error about missing symbols (that live in libHSbase) when loading libHSrts. I then tried rebuilding libHSrts against libHSbase... and then I got the first error again, about missing symbols (that live in libHSrts) when loading libHSbase, while loading libHSrts. I didn't try also rebuilding libHSbase against libHSrts, because at that point the base library would be dependent on the RTS flavor, which we don't really want. It appears based on these tests that the Android dynamic loader only looks for missing symbols in a shared library in the dependencies of that library in a depth-first fashion, while the Linux ld.so seems to do something more like loading every (direct and indirect) dependency of the executable first and then resolving symbol references between them all at once. I didn't fully characterize the Android loader behavior though, and it's possible that if we could remove the references from libHSrts to libHSbase and libHSghc-prim, then we could list libHSrts first among the dependencies of an executable, and hopefully once libHSrts has been loaded, the loader would then use its symbols to resolve references when loading the libHSghc-prim and libHSbase libraries, even without those libraries having NEEDED entries for libHSrts. Removing the references from libHSrts to libHSbase/libHSghc-prim should be fairly mechanical if we can use constructors in the latter libraries (basically emulate dynamic linking by adding a global variable to libHSrts which is a struct holding the addresses of the base/ghc-prim symbols it needs, and have constructors in those libraries fill in those addresses on load). If that doesn't work, another possibility may be to statically link libHSrts (or whatever part of it we need) into the executable and export libHSrts's dynamic symbols so that they can be used by libHSghc-prim (or other Haskell libraries). I also haven't tested whether this kind of mutual recursion between an executable and its shared library dependencies works on Android. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10324> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8287: exploring calling convention changes and related engineering for 7.10
by GHC 25 Jan '16

25 Jan '16

#8287: exploring calling convention changes and related engineering for 7.10 ------------------------------------+------------------------------------- Reporter: carter | Owner: Type: task | Status: new Priority: high | Milestone: 7.10.1 Component: Compiler | Version: 7.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Rocket Science | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- I'm creating this as a master ticket for systematically exploring and benchmarking (possibly breaking) changes to the GHC calling convention. This work will initially focus on x86/x86_64, but may have larger scope. Goals: 1. if possible, improve performance systematically for code run on recent CPU micro architecture generations (eg sandybridge, haswell, and future extensions like Knights Landing and Skylake), ideally without pessimizing code on other or older x86_64 micro-architectures 2. Try to bring native and llvm codegens closer to feature parity (or at the very least, do not widen their capability gap) 3. a few other pieces too, will amend this ticket as ideas / plans clarify -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8287> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10433: Fix load/store barriers in pre-ARMv7 builds
by GHC 25 Jan '16

25 Jan '16

#10433: Fix load/store barriers in pre-ARMv7 builds -------------------------------------+------------------------------------- Reporter: | Owner: simonmar thoughtpolice | Status: new Type: bug | Milestone: 7.12.1 Priority: high | Version: Component: Runtime | Operating System: Unknown/Multiple System | Type of failure: None/Unknown Keywords: | Blocked By: Architecture: arm | Related Tickets: #10244 Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- As pointed out in #10244 and Phab:D894, the fix we committed for this problem isn't 100% correct - from 753b156dc6b0c38b106c390952750fb800bf27e7: {{{ #elif arm_HOST_ARCH && defined(arm_HOST_ARCH_PRE_ARMv7) // TODO FIXME: This case probably isn't totally correct - just because we // use a pre-ARMv7 toolchain (e.g. to target an old Android device), doesn't // mean the binary won't run on a newer ARMv7 system - in which case it // needs a proper barrier. So we should rethink this // - Reid __asm__ __volatile__ ("" : : : "memory"); }}} This is a reminder to fix this. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10433> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9149: Description of openFileBlocking is wrong
by GHC 25 Jan '16

25 Jan '16

#9149: Description of openFileBlocking is wrong ------------------------------------+------------------------------------- Reporter: edsko | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Documentation | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- The documentation states: ''Like openFile, but opens the file in ordinary blocking mode. This can be useful for opening a FIFO for reading: if we open in non-blocking mode then the open will fail if there are no writers, whereas a blocking open will block until a writer appears. '' but I think this is the wrong way around, and should be: ''Like openFile, but opens the file in ordinary blocking mode. This can be useful for opening a FIFO for writing: if we open in non-blocking mode then the open will fail if there are no readers, whereas a blocking open will block until a reader appears. '' -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9149> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10751: Implement Phase 1 of MonadFail Proposal (MFP)
by GHC 25 Jan '16

25 Jan '16

#10751: Implement Phase 1 of MonadFail Proposal (MFP) -------------------------------------+------------------------------------- Reporter: hvr | Owner: Type: task | Status: new Priority: normal | Milestone: 7.12.1 Component: Compiler | Version: 7.10.2 Keywords: report- | Operating System: Unknown/Multiple impact | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- See also - https://www.reddit.com/r/haskell/comments/3a1o06/monadfail_proposal_mfp/ TODO: At this point this ticket is mostly a reminder that stuff may need to be done for GHC 7.12 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10751> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11248: Ambiguous Types with Constraint Synonyms
by GHC 25 Jan '16

25 Jan '16

#11248: Ambiguous Types with Constraint Synonyms -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following code fails to compile: {{{ {-# LANGUAGE DataKinds, TypeOperators, TypeFamilies, KindSignatures, ConstraintKinds #-} import GHC.TypeLits type a / b = FDiv a b type a ** b = FMul a b type family FDiv a b where FDiv 11648 128 = 91 type family FMul a b where FMul 64 91 = 5824 type family FGCD a b where FGCD 128 448 = 64 FGCD 128 5824 = 64 type family FLCM a b where FLCM 128 5824 = 11648 data CT (m :: Nat) (m' :: Nat) type H0 = 128 type H1 = 448 type H0' = 11648 type H1' = 5824 main' = let x = undefined :: CT H0 H0' in foo x :: CT H1 H1' foo x = bug x type Ctx2 e r s e' r' = (e ~ FGCD r e', r' ~ FLCM r e', e ~ FGCD r s) type Ctx1 e r s e' r' = (Ctx2 e r s e' r', e' ~ (e ** (r' / r))) bug :: (Ctx1 e r s e' r') => CT r r' -> CT s s' bug = undefined }}} with the error {{{ Could not deduce ((~) Nat (FGCD r e'0) (FGCD r s)) ... The type variable ‘e'0’ is ambiguous When checking that ‘foo’ has the inferred type foo :: forall (r :: Nat) (s :: Nat) (s' :: Nat) (e' :: Nat). ((~) Nat (FGCD r s) (FGCD r e'), (~) Nat (FMul (FGCD r s) (FDiv (FLCM r e') r)) e') => CT r (FLCM r e') -> CT s s' Probable cause: the inferred type is ambiguous }}} However, if I move the `e' ~ ...` constraint from `Ctx1` to `Ctx2` or to the context of `bug`, it compiles as expected. Somehow, GHC misses the constraint when it is in `Ctx1`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11248> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11056: Need to generate Typable info for promoted data constructors
by GHC 25 Jan '16

25 Jan '16

#11056: Need to generate Typable info for promoted data constructors -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: high | Milestone: 8.0.1 Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Trac #10052 isn't relevant for HEAD, because HEAD generates implicit `Typeable` bindings for everything. But in testing {{{ {-# LANGUAGE PolyKinds, DataKinds #-} module Foo where import Data.Typeable data T = A | B Int bar :: TypeRep bar = typeRep (Proxy :: Proxy '[True]) }}} I discovered that we aren't generating tycon-rep bindings for promoted data constructors. As a result we get {{{ GHC error in desugarer lookup in Foo: Can't find interface-file declaration for variable tc'[] Probable cause: bug in .hi-boot file, or inconsistent .hi file Use -ddump-if-trace to get an idea of which file caused the error }}} In particular `TcTypeable.mkTypeableBinds` doesn't generate the bindings for the `tc'[]` and `tc':`. It's not hard to fix this, but it will add more top-level declarations for each data type decl. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11056> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10560: -f and -O options interact in non-obvious, order dependent ways
by GHC 25 Jan '16

25 Jan '16

#10560: -f and -O options interact in non-obvious, order dependent ways -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- The `-O[012]` and `-f*` flags are both implemented by manipulating a set of flags in `DynFlags`. These manipulations occur in the order that the flags occur in the command line. This leads to some rather surprising behavior. For instance, if the user wants to compile with optimization but specifically wants to disable specialisation, they might enter `-fno- specialise -O1`. This, however, would not do what the user expects as `-O1` implies `-fspecialise`, overriding the first flag. This is surprising and poorly documented behavior at best. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10560> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11306: Do not generate warning in `do` when result is of type `Void`.
by GHC 25 Jan '16

25 Jan '16

#11306: Do not generate warning in `do` when result is of type `Void`. -------------------------------------+------------------------------------- Reporter: diatchki | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- GHC generates warnings when `do` notation statements discard their result, without an explicit `_ <- `, unless the discarded result is of type `()`. Another type which is used to indicate "no result", is `Void` from `Data.Void`. There are no meaningful values of this type, so we should also skip the warnings for such situations. Here is an example: {{{#!hs import Data.Void f :: IO Void f = undefined main :: IO () main = do f return () }}} GHC output: {{{ test.hs:7:11: Warning: A do-notation statement discarded a result of type ‘Void’ Suppress this warning by saying ‘_ <- f’ or by using the flag -fno-warn-unused-do-bind }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11306> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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